The present invention pertains especially to gate valves, especially through-conduit type gate valves, and seats therefor, although certain principles of the invention may also be applicable to other types of valves. A gate valve assembly typically includes a valve body defining a longitudinal flowway and a valve element mounted in the valve body and movable tranverse to the flowway. In through-conduit gate valves, this valve element, or gate, has a solid portion of sufficient dimension to block off and close the flowway when it is aligned therewith. The gate also includes a bore or port oriented parallel to the flowway. By moving the gate in the aforementioned manner, the solid portion thereof can be displaced from alignment with the flowway, and the port moved into alignment with the flowway to open the valve. Such valves also typically include annular valve seats mounted in the valve body, coaxially with the flowway, on opposite sides of the gate, for sealing engagement with the latter.
In some types of gate valve assemblies, the valve seats are fixedly mounted in the valve body. The gate of such a valve assembly is formed in multiple parts which, when the valve is closed, are expanded longitudinally against the valve seats to form tight seals. In other valve assemblies, so called "floating seats" are employed. These seats are permitted limited axial play with respect to the valve body, and it is by virtue of such movement that they sealingly engage the gate. Floating seats are a virtual necessity for proper upstream sealing where the gate is of the "slab" or simple, straight-sided, one piece variety.
One of the advantages of through-conduit gate valve assemblies for oil field use, or other uses in which the fluid being handled may contain abrasive material, is that the gate periodically wipes the sealing faces of the seats as it is moved between its open and closed positions, and even when in the open position, remains in contact with the sealing areas of the seats, thereby protecting them. However, these types of valves also involve disadvantages, some of which are inversely related such that prior attempts to alleviate one of the problems would aggravate the other.
One such problem revolves around the force required to open the valve. This force is dependent on the sliding friction forces between the gate and seats. Even in those valve assemblies which are designed to seal only at the downstream seat, the operating force is a function of the outer diameter of the annular sealing area of the downstream seat. Where the valve also provides a secondary seal at the upstream seat, the operating force is further magnified. Logically, a reduction in the outer diameter of the seat's sealing area would correct this problem. However, too much reduction in this diameter will magnify a different problem revolving around the need for adequate bearing area. Since the sealing area and bearing area of the conventional valve seat are coextensive, a reduction in the sealing area also reduces the bearing area. This is particularly undesirable in high-pressure valves, especially since the applicable industry standards require the use of relatively soft metals. It can be seen that, particularly with such relatively soft metals, if a high-pressure force is distributed over only a very small bearing area, galling and material failure can occur.
A third common problem with these types of valve assemblies is related to the fact that the valve body defines cavities adjacent the flowway for receipt of the gate. During normal operation with the valve open, pressurized fluid may enter these cavities. In any event, when the valve is moved to its closed position, this pressurized fluid will enter the cavities and may become trapped in the valve body. More particularly, this trapping occurs in assemblies with floating seats where the diameter of the seal between the seat and valve body is less than the diameter of the sealing area of the seat against the gate. With this relationship in diameters, the pressure already within the cavities in the valve body will urge the seats into even tighter engagement with the gate, which increases the force necessary to reopen the valve. In the meanwhile, with the valve still closed, the trapped fluid within the valve body makes it susceptible to explosion, representing an extremely dangerous situation.